Typically, such wind turbines have been controlled in generator speed to deliver maximum power to the power grid and certain fixed maximum and minimum speed limits have been imposed in order to limit the load on the turbine components, e.g., the converter, drive train, generator, etc.
U.S. Pat. No. 5,729,118 discloses a variable speed induction generator-motor with controllable excitation frequency, in which the slip frequency is kept constant during variations of the power grid frequency.